Photoinitiation systems for free radical polymerization reactions

ABSTRACT

Acyloin urethans can be employed to effect photopolymerization of free radical-polymerizable compositions in inert and oxygen-containing atmospheres.

This is a division of application Ser. No. 950,970; filed Oct. 13, 1978,now U.S. Pat. No. 4,224,454.

This invention relates to energy-curable compositions. Moreparticularly, the invention relates to photopolymerization of freeradical-polymerizable materials in inert and oxygen-containingatmospheres. The invention especially relates to acyloin urethancompounds and to photopolymerizable compositions containing suchcompounds.

The paint, coating and ink industries have accelerated the developmentof photocurable compositions, which contain substantially no inertvolatile solvents, to meet increasing pressures to reduce solventemissions and to conserve energy. Typically, such systems are composedof various reactive components which cure through a photoinitiated freeradical polymerization mechanism. The reactive ingredients include lowmolecular weight polymeric materials usually referred to as oligomer(s),and monomeric materials, and can include mixtures of such materials.Formulations containing such materials generally contain photoinitiationcompounds which accelerate the polymerization rate on exposure toactinic radiation. The function of such photoinitiation compounds is toabsorb impinging energy in an amount sufficient to energize the compoundto an electronically excited state which is effective to induce freeradical polymerization of the reactive oligomeric and monomericmaterials by one or more of several mechanisms, including direct energytransfer to a reactive site, by the formation of free radicals in abimolecular process such as hydrogen abstraction, through the formationof a donor-acceptor complex between monomer and additive leading toionic or radical species, and by decomposing directly to free radicalscapable of initiating polymerization. The photoinitiator componentshould not adversely affect the viscosity, stability, odor or color ofthe photocurable compositions or the cured product and should benon-toxic and yield products after exposure to actinic radiation whichare non-toxic, colorless and without objectionable odor. Thephotoinitiator component preferably should be inexpensive, be liquid forease of handling and blending, as well as compatible with theformulation. Most importantly, the photoinitiator component must beeffective to initiate polymerization in a controlled efficient mannerand not at the same time produce products which could be effective toprematurely terminate the polymerization reaction.

A large number of compounds are known which are effective to promotefree radical photopolymerization reactions. These include the benzoinethers of primary and secondary alcohols, aromatic glyoxals such asphenyl glyoxal, diketones such as 1-phenyl-butane-1,2-dione,acetophenones such as 2,2-diethoxyacetophenone, and halogenatedcompounds such as polychlorinated diphenyl resins, inter alia.Notwithstanding the plethora of known compounds which are currentlyindustrially acceptable as initiators for photopolymerization reactions,new and improved photoinitiators are required to meet increasinglysophisticated demands resulting from the increasing acceptance ofphotopolymerization techniques due to the inherently lower equipmentcosts, reduction of volatile emissions, and reduced energy consumptionwhich attend their use.

The present invention provides a new class of acyloin derivatives whichare effective to promote free radical photopolymerization reactions. Theinvention also provides novel photocatalyst systems which are effectiveto promote free radical photopolymerizations under inert atmospheres andin oxygen-containing enviornments. Further, the invention provides novelphotocurable compositions which can be readily cured in either an inertatmosphere or in an oxygen-containing atmosphere by exposure to actinicradiation. The invention also provides processes for effectingphotopolymerization of free radical-polymerizable polymeric andmonomeric materials, including mixtures of such materials, to provideproducts having high gloss and low gloss finishes.

More particularly, there is provided in accordance with this inventionnovel acyloin urethan compounds which have been found effective topromote free radical photopolymerization reactions, such acyloin urethancompounds having the formulas ##STR1## wherein: A¹ and A² are the sameor different and each is an unsubstituted or substituted benzenoid ornon-benzenoid aromatic radical;

R⁰ is hydrogen, or an aliphatic, cycloaliphatic or aromatic radicalcontaining from 1 to 9 carbon atoms, or --OR², or ##STR2## R¹ ishydrogen, or an aliphatic, cycloaliphatic or aromatic radical containingfrom 1 to 9 carbon atoms, or --R³ --O--R⁴, or ##STR3## R² is hydrogen,or an aliphatic, cycloaliphatic or aromatic radical having from 1 to 9carbon atoms, or --R³ --O--R⁴, or ##STR4## R³ is a divalent aliphatic,cycloaliphatic or aromatic radical containing from 1 to 9 carbon atoms;

R⁴ is hydrogen, or an aliphatic, cycloaliphatic, or aromatic radicalhaving from 1 to 9 carbon atoms, or ##STR5## Q is the monovalent orpolyvalent residue of a monomeric aliphatic, cycloaliphatic or aromaticisocyanate compound having at least one isocyanate group or of anisocyanate-functional polymeric material characterized by the presenceof at least one isocyanate group, which residue remains after reactionof all isocyanate groups of such monomeric compounds and polymericmaterials with one or more saturated or unsaturated compounds containingat least one isocyanate-reactive active hydrogen atom;

X is selected from the group consisting of --O--, --S--, and >NR⁵,wherein R⁵ is hydrogen or an alkyl group having from 1 to 9 carbonatoms;

Y is hydrogen or the organic residue of a saturated or unsaturatedorganic compound containing at least one isocyanate-reactive activehydrogen atom, wherein at least one such active hydrogen atom has beenremoved by reaction with an isocyanate moiety;

m is a number in the range from 1 to f, where f is the total number ofisocyanate groups per molecule of isocyanate compound; and

n is equal to f-m.

The groups A¹ and A² individually are mononuclear or polynuclearbenzenoid moieties having from 6 to 14 nuclear carbon atoms ornon-benzenoid cyclic moieties which are recognized in the art aspossessing the characteristics of a benzenoid aromatic moiety, suchnon-benzenoid moieties having from 4 to 13 nuclear or ring carbon atomsand up to two atoms of oxygen, sulfur, nitrogen or combinations thereof,which benzenoid and non-benzenoid cyclic moieties can carry one or moresubstituents selected from alkyl having from 1 to 12 carbon atoms,alkenyl having from 2 to 8 carbon atoms, phenyl, aralkyl or alkarylhaving from 7 to 15 carbon atoms, alkoxy having from 1 to 10 carbonatoms, phenoxy, alkylthio having from 1 to 10 carbon atoms, phenylthio,alkanoyl having from 2 to 12 carbon atoms and halogen. The groups A¹ andA² together can form a fused aromatic ring system or can be linkedtogether by a direct carbon-to-carbon link or through a divalentaliphatic chain having from 1 to 6 carbon atoms. Representative A¹ andA² groups, including fused and linked ring systems, include, withoutlimitation thereto, phenyl, cumenyl, mesityl, tolyl, xylyl, naphthyl,fluorenyl, methoxyphenyl, chlorophenyl, thienyl, pyrolyl,benzo[b]thienyl, furyl, pyridyl, isobenzofuranyl, pyrazinyl,thiathrenyl, naphthylene, anthralene, biphenylene andmethylenebisphenylene.

The novel acyloin urethan compounds of the present invention can bebroadly described as the reaction product of an aromatic monoketone anda monomeric or polymeric compound having at least one isocyanate groupand including the reaction product of an aromatic monoketone, amonomeric or polymeric compound having at least two isocyanate groupsand a compound, preferably having at least one olefinically unsaturatedmoiety, containing at least one isocyanate-reactive active hydrogenatom.

More particularly the novel acyloin urethan compounds of this inventioncomprise the reaction product of at least one aromatic acyloin compoundhaving the formula ##STR6## wherein A¹, A², R⁰ and R¹ are as previouslydescribed; and at least one monomeric or polymeric isocyanate compoundhaving the formula

    Q--NCO.sub.f,                                              V.

wherein Q and f are as previously defined; and, optionally, at least onecompound, different from said acyloin compound, having at least oneactive hydrogen moiety selected from the group consisting of --OH, --SH,--NH, and >NH and preferably having at least one unit of olefinicunsaturation. The reactants are preferably employed in chemicallyequivalent amounts, that is, the amount needed to furnish one activehydrogen atom for each isocyanate group present. The reaction iseffected at standard conditions for the isocyanate-active hydrogenreaction and is continued to an end point corresponding to essentially100 percent depletion of isocyanate functions. The end point of thereaction is readily attainable by any of several well known analyticalmethods, such as by titration. To ensure substantially complete reactionof all isocyanate functions, it can be advantageous to employ theacyloin compound in a slight excess, for example, 1 equivalent excess.When Y is other than hydrogen, the saturated or unsaturated compoundhaving at least one active hydrogen atom can be prereacted with anexcess of polyisocyanate to obtain an isocyanate-functional adduct orprepolymer which can then be reacted with the acyloin compound.Alternatively, the acyloin compound can be prereacted with excesspolyisocyanate and resulting isocyanate-functional aromatic acyloinurethan can be reacted with the active hydrogen compound, the lattercompound preferably being present in a slight excess. Currently, it ispreferred to combine all the reactants simultaneously and, afterreaching the theoretical end point of the reaction to add a minor amountof a monoalcohol having from 1 to 4 carbon atoms to ensure completereaction of all isocyanate groups. The reaction can be accomplished inthe presence or absence of diluents. Preferred diluents includealiphatic, cycloaliphatic and aromatic compounds such as heptane,cyclohexane and toluene, but other diluents such as methyl ethyl ketonecan be employed. Currently, it is preferred to effect the reaction inthe substantial absence of any diluent compound other than a slightexcess of acyloin compound.

Examples of suitable aromatic ketones which can be employed in formingacyloin urethans in accordance with this invention are benzoin,α-methyl-benzoin, α-methylol-benzoin methyl ether, α-methylol-benzoinisopropyl ether, 2,2'-dichlorobenzoin, 4,4'-dichlorobenzoin,4,4'-dimethoxy benzoin, α-naphthoin and 2,2'-furoin. Substantially anyaromatic acyloin compound having the formulas III or IV, includingmixtures thereof can be employed in preparing the herein describedaromatic acyloin urethan compounds.

Illustrative of the monomeric and polymeric monoisocyanates andpolyisocyanates employed in preparing the new acyloin urethanes arehexyl isocyanate, cyclohexyl isocyanate, phenyl isocyanate, toluenediisocyanate, 4,4'-diphenyl diisocyanate, 4,4'-diphenylene methanediisocyanate, dianisidine diisocyanate, 1,5-naphthalene diisocyanate,4,4'-diphenyl ether diisocyanate, p-phenylene diisocyanate, trimethylenediisocyanate, hexamethylene diisocyanate, nonamethylene diisocyanate,octadecamethylene diisocyanate, 2-chloropropane diisocyanate,3-(dimethylamine)pentane diisocyanate, transvinylene diisocyanate,isocyanate prepolymers obtained by reacting excess monomericpolyisocyanates having at least two isocyanate groups with polyaminescontaining primary and secondary amine groups, polythiols having atleast two mercaptan groups and polyols such as poly(alkyleneoxide)glycols, polyesters, polyetheresters, polycaprolactones and alkaneand alkene polyols having from 2 to 6 hydroxy groups.

Saturated and unsaturated compounds containing at least oneisocyanate-reactive active hydrogen atom which can be employed inpreparing acyloin urethans in accordance with the invention include, forexample, methanol, isobutanol, 2-ethylhexanol, ethyl amine, ethylmercaptan, allyl alcohol, methallyl alcohol, 1,4-butane diol, monovinylether, crotonol, ethylene glycol monoacrylate and monomethacrylate andmonomeric acrylic compounds having the formula ##STR7## wherein R⁶ ishydrogen, an alkyl radical having from 1 to 4 carbon atoms, CH═CH-- orhalogen; R⁷ is a divalent organic radical selected from the groupconsisting of alkylene having from 1 to 8 carbon atoms, alkyleneoxyunits derived from poly(alkylene oxide)polyols having from 1 to 9 carbonatoms separating each pair of oxygen atoms, including mixtures thereof,phenylene and naphthylene; and X and Y are as previously defined.Poly(alkylene oxide)polyols which can be employed to form compoundshaving the formula VI are well known. Compounds having the formula VIare currently preferred.

The novel aromatic acyloin urethan compounds of the invention areparticularly suitable for the photopolymerization of reactive monomers,oligomers and mixtures thereof, under inert atmospheres, such asnitrogen, when used alone. Generally, the acyloin urethan compounds willbe employed in amounts in the range from 0.01 to 10, preferably 0.05 to7, percent by weight, based on total weight of reactive constituents.

The novel photocatalyst systems of the present invention comprise anadmixture of (1) at least one aromatic acyloin urethan compound havingthe formulas I and II and (2) at least one compound which promotes freeradical polymerization through bimolecular photochemical reactions ofthe energy donor or energy transfer type, the hydrogen abstraction type,or by the formation of a donor-acceptor complex with monomers oradditives leading to ionic or radical species. Such photocatalystsystems are particularly suited to effect photopolymerization inoxygen-containing atmospheres. It is well known that the presence ofoxygen has an inhibiting effect on free radical polymerizationreactions. The novel photocatalyst systems of this inventionsubstantially negate such oxygen inhibition.

Compounds (2) which are effective to promote free radical additionpolymerization through bimolecular photochemical reactions of the energydonor or transfer type or hydrogen abstraction type or by formation of adonor-acceptor complex with monomers or additives leading to ionic orradical species have been described as photosensitizers by at least onepatentee, see Gruber U.S. Pat. No. 4,017,652, and, for the purpose ofestablishing some measure of consistency with respect to nomenclature,that description will be followed herein. With respect tophotopolymerization processes, photosensitizers are not good initiatorsper se, but do readily absorb photon to produce an excited moleculewhich then acts through energy transfer, hydrogen abstraction orformation of a donor-acceptor complex with a second molecule to producefree radicals which are capable of initiating additional polymerizationreactions.

Particularly preferred photosensitizers are aromatic ketones andaromatic aldehydes which can exist in a triplet state, especially suchketones and aldehydes which have a triplet energy in the range from 35to 85, preferably 42 to 72, kilocalories per mole. Such photosensitizersare described in Gruber U.S. Pat. No. 4,017,652 and Osborn et al U.S.Pat. No. 3,759,807, the disclosures of both patents being incorporatedherein by reference. The latter patent discloses representativeactivators which, in combination with photosensitizer compounds, act toform donor-acceptor complexes.

In forming the novel photocatalyst systems of this invention, thephotosensitizers component (2) will generally comprise from 25 to 90,preferably 35 to 80, and especially 45 to 75, percent by weight of thephotocatalyst system. The aromatic acyloin urethan component (1) isusually present in an amount in the range of 10 to 75, preferably 20 to65, and especially 25 to 55, percent by weight of the photocatalystsystem. The amount of photocatalyst system present in photopolymerizablecompositions according to the invention is at least an effective amount,with the photosensitizer being usually present in an amount in the rangefrom 0.01 to 50, preferably 0.1 to 15, parts by weight per 100 parts byweight of reactive constituents; and the aromatic acyloin urethancompounds being usually present in an amount in the range of 0.01 to 10,preferably 0.05 to 7, parts by weight per 100 parts by weight ofreactive constituents.

Photocurable compositions in accordance with this invention which arecurable by actinic radiation comprise, in addition to the novel aromaticacyloin urethan photoinitiators or the novel photocatalyst systems ofthe invention, at least one reactive monomer, at least one reactivepolymer or a mixture of such polymers and monomers, with the lattermixtures being currently preferred. Preferably, reactive polymericmaterials comprise at least one relatively low molecular weight polymeror oligomer.

Reactive oligomers which are preferentially employed in the photocurablecompositions of this invention can include substantially any polymericmaterial characterized by the presence of at least one, preferably atleast two, ethylenically unsaturated unit(s), and which is curablethrough a free radical-induced polymerization mechanism. Such polymericmaterials will exhibit a molecular weight of at least 600, andpreferably in the range of 900 to 4500, and preferably will have from0.5 to 3 units of α,β-olefinic unsaturation per 1000 units of molecularweight. Representative of such materials are vinyl, acrylic, substitutedacrylic, allylic, mercapto, fumaric, maleic and the like compoundshaving at least one unit of ethylenic unsaturation, includingethylenically unsaturated polyesters, polyethers, polyacrylates andsubstitute acrylates, epoxies, urethanes, silicones, amines, polyamides,and the like. A preferred class of polymeric materials includes theacrylated resins, such as acrylated silicone oil, acrylated polyesters,acrylated polyethers, acrylated polyurethanes, acrylated polyamides,acrylated polycaprolactones, acrylated soybean oil, acrylic andsubstituted acrylic resins, acrylated epoxies and acrylated urea resins,with acrylated polyurethane resins being particularly preferred. Suchethylenically unsaturated materials, including their manufacture, arewell known, see Burlant et al U.S. Pat. No. 3,509,234, Smith et al U.S.Pat. No. 3,700,645 and Boranian et al U.S. Pat. No. 3,924,023.

A particularly preferred class of polymeric materials compriseunsaturated urethane and analogous to urethane resins which arecharacterized by the presence of at least one ethylenically unsaturatedunit having the structure --C═C, said unsaturated resins comprising thereaction product of:

(i) at least one organic isocyanate compound characterized by thepresence of at least two reactive isocyanate groups;

(ii) from about 30 to 100 mol percent of at least one polymeric materialcharacterized by the presence of at least two isocyanate-reactive activehydrogen groups;

(iii) from about 70 to zero mol percent of at least one monomericchain-extending compound characterized by the presence of at least twoisocyanate-reactive active hydrogen groups; and

(iv) at least one addition-polymerizable unsaturated monomeric compoundhaving a single isocyanate-reactive active hydrogen group;

the mol percents of (ii) and (iii) being based on total mols of (ii) and(iii);

said isocyanate compound (i) being present in an amount sufficient toprovide an NCO:active hydrogen ratio greater than 1:1, preferably atleast 1.05:1, and more preferably in the range 2.3-5:1, with respect tothe active hydrogen groups of (ii) and (iii);

said addition-polymerizable unsaturated monomeric compound (iv) beingpresent in an amount sufficient to provide at least one molar equivalentof active hydrogen group per mol of available isocyanate moiety. Suchpreferred unsaturated resins will have a residual reactive isocyanatemoiety, based on total weight of the resin, of not more than one,preferably not more than 0.1, percent by weight. The ethylenicallyunsaturated unit is preferably a terminal group having the structure CH₂═CH--. Such resins have the further characteristic features

(a) the polymerizable ethylenically unsaturated group is separated fromthe main or backbone carbon-carbon chain by at least one, preferably atleast two, urethane or analogous group(s) or combination of such groups;

(b) a molecular weight of at least 600, preferably 900 to 4500; and

(c) the presence of 0.5 to 3 ethylenically unsaturated units per 1000units of molecular weight.

An especially preferred group of reactive oligomers comprises thereaction product of

(i) at least one organic isocyanate compound having at least twoisocyanate groups;

(ii) at least one poly(alkylene oxide)polyol; and

(iii) at least one unsaturated addition-polymerizable monomeric compoundhaving a single isocyanate-reactive active hydrogen group;

there being present an excess of isocyanate compound with respect to thehydroxyl groups of said poly(alkylene oxide)polyol;

said unsaturated addition-polymerizable monomeric compound having asingle isocyanate-reactive active hydrogen group being present in anamount sufficient to provide at least one molar equivalent of activehydrogen group with respect to isocyanate reactivity.

Active hydrogen-containing precursors which can be employed in preparingthe preferred ethylenically unsaturated reactive oligomers can be linearor branched and include any polymeric material having at least twoisocyanate-reactive active hydrogen groups per molecule as determined bythe Zerewitinoff method. Representative active hydrogen-containingpolymeric compounds include polyethers, such as polyethylene glycol andpolytetramethylene glycol; hydroxy-terminated polyalkylene esters ofaliphatic, cycloaliphatic and aromatic diacids; esters of polyhydricalcohols and hydroxy fatty acids; alkyd resins containing hydroxyl endgroups; hydroxyl-terminated polybutadiene resins; hydroxylated acrylicand substituted acrylic resins; hydroxyl-terminated vinyl resins;polycaprolactones; polythiols; polyamine and polyamide resins and thelike. Currently, hydroxyl-containing compounds are preferred.

Organic isocyanate compounds suitable for use in forming the preferredunsaturated resins in accordance with the invention can be any organicisocyanate compound having at least two reactive isocyanate groups.Included within the purview of such isocyanate compounds are aliphatic,cycloaliphatic, and aromatic polyisocyanates as these terms aregenerally interpreted in the art. Thus, it will be appreciated that anyof the known polyisocyanates such as alkyl and alkylene polyisocyanates,cycloalkyl and cycloalkylene, polyisocyanates, and aryl and arylenepolyisocyanates, including variants thereof, such as alkylenecycloalkylene and alkylene arylene polyisocyanates, can be employed.Suitable polyisocyanates include, without limitation,tolylene-2,4-diisocyanate,2,2,4-trimethylhexamethylene-1,6-diisocyanate,hexamethylene-1,6-diisocyanate, diphenylmethane-4,4'-diisocyanate,triphenylmethane-4,4',4"-triisocyanate, polymethylene poly(phenylisocyanate), m-phenylene diisocyanate, 2,6-tolylene diisocyanate,1,5-naphthalene diisocyanate, naphthalene-1,4-diisocyanate,diphenylene-4,4'-diisocyanate, 3,3'-bi-tolylene-4,4'-diisocyanate,1,4-cyclohexylene dimethylene diisocyanate, xylene-1,4-diisocyanate,cyclohexyl-1,4-diisocyanate, 4,4'-methylene-bis(cyclohexyl diisocyanate,3,3'-diphenyl methane-4,4'-diisocyanate, isophorone diisocyanate, dimerisocyanates such as the dimer of tolylene diisocyanate, and the productobtained by reacting trimethylol propane and 2,4-tolylene diisocyanatein a molar ratio of 1:3. Currently, aliphatic and cycloaliphaticdiisocyanates are preferred.

Essentially any monomeric compound having at least twoisocyanate-reactive active hydrogen groups which is known to or can beexpected to function as a chain-extender to increase molecular weight,introduce chain-branching, affect flexibility and the like in reactionsbetween isocyanate compounds and compounds containing active hydrogengroups can be employed in forming the preferred unsaturated resins ofthe invention. Such chain-extending compounds are well known in the artand require no detailed elaboration. Preferably, the active hydrogengroups of such chain-extending compounds will be selected from amonghydroxyl, thiol, primary amine and secondary amine, including mixturesof such groups, with hydroxyl and primary amine being currentlypreferred. The chain-extending compounds will generally have molecularweights of less than 25, and preferably between 62 and 225. Especiallypreferred chain-extending compounds include aliphatic diols free ofalkyl substitution and aliphatic triols having from 2 to 14 carbonatoms. Representative chain-extending compounds include water, ethyleneglycol, 1,3-propane diol, 1,4-butane diol, 1,6-hexane diol, trimethylolpropane, triethylene glycol, glycerol, 1,2-propane-bis(4-cyclohexylamine), methane-bis(4-cyclohexyl amine), N,N'-dimethyl-o-phenylenediamine, 1,3-propane dithiol, monoethanol amine, and amino ethylmercaptan.

Suitable addition-polymerizable monomeric compounds having a singleethylenically unsaturated unit and a single isocyanate-reactive hydroxylactive hydrogen group which can be used in the preferred compositions ofthis invention include 2-hydroxyethyl acrylate, 3-hydroxypropylacrylate, 4-hydroxybutyl acrylate, 8-hydroxyoctyl acrylate,12-hydroxydodecanyl acrylate, 6-hydroxyhexyl oleate, hydroxy neopentylacrylate, hydroxyneopentyl linoleate,hydroxyethyl-3-cinnamyloyloxypropyl acrylate, hydroxyethyl vinyl ether,and the corresponding methacrylates, and allyl alcohol.

The preferred unsaturated resins of the invention can be prepared by anyof several reaction routes. For example, the isocyanate compound, thepolymeric material having at least two active hydrogen groups, theaddition-polymerizable monomeric compound having a single ethylenicallyunsaturated group and a single isocyanate-reactive active hydrogen groupand, when used, the chain-extending compound can be simultaneouslyreacted together. Currently, it is preferred to form the unsaturatedresins in two or more steps comprising (1) reacting the isocyanatecompound, the polymeric material, and, if used, the chain-extendingcompound to provide an isocyanate-functional prepolymer and (2) reactingthe prepolymer with the addition-polymerizable unsaturated monomericcompound having a single isocyanate-reactive active hydrogen group. Thereaction is terminated at the desired state of viscosity, which willgenerally correspond to a molecular weight of at least 600, preferably900 to 4500, which is usually a function of an end-use requirement. Anyexcess isocyanate moieties can be capped if desired or necessary by theaddition of monofunctional chain-terminating agents, such asmonoalcohols and monoamines, preferably having from one to 4 carbonatoms, and morpholine. Regardless of the process employed, it ispreferred to conduct the reaction in its entirely in the presence of adiluent phase which is copolymerizable with the unsaturated resinproduct but is inert with respect to the manufacture of the resin.

Reactive diluent systems which can be employed in theaddition-polymerizable compositions of this invention include any ofsuch systems which have been or are being used for this purpose.Broadly, suitable reactive diluent systems comprise at least oneunsaturated addition-polymerizable monomer which is copolymerizable withthe unsaturated resin. The reactive diluent can be monofunctional orpolyfunctional. A single polyfunctional diluent can be used, as canmixtures thereof, or a combination of one or more monofunctionalreactive diluents can be used. Such combinations of mono- andpolyfunctional reactive diluents are currently preferred. Generally, thereactive diluent system will comprise from about 10 to about 75,preferably about 25 to about 50, weight percent, based on total weightof unsaturated resin and reactive diluent, of the addition-polymerizablecompositions of the invention. Particularly preferred reactive diluentsare unsaturated addition-polymerizable monofunctional monomericcompounds selected from the group consisting of esters having thegeneral formula ##STR8## wherein R⁸ is hydrogen or methyl and R⁹ is analiphatic or cycloaliphatic, preferably alkyl or cycloalkyl, grouphaving from 6 to 18, preferably 6 to 9 carbon atoms. Representative ofsuch preferred reactive monomeric diluents, without limitation thereto,are hexyl acrylate, cyclohexyl acrylate, 2-ethylhexyl acrylate, octylacrylate, nonyl acrylate, stearyl acrylate, and the correspondingmethacrylates. It is preferred that at least 50 percent by weight of thereactive diluent comprise one or more of these preferred esters.Illustrative of other reactive monofunctional and polyfunctionalmonomeric diluents which can be employed are styrene, methylmethacrylate, butyl acrylate, isobutyl acrylate, 2-phenoxy acrylate,ethoxyethoxyethyl acrylate, 2-methoxyethyl acrylate,2-(N,N'-diethylamino)-ethyl acrylate, the corresponding methacrylates,acrylonitrile, methyl acrylonitrile, methacrylamide, neopentyl glycoldiacrylate, ethylene glycol diacrylate, hexylene glycol diacrylate,diethylene glycol diacrylate, trimethylol propane triacrylate,pentaerythritol di-, tri-, or tetra-acrylate, the correspondingmethacrylates, vinyl pyrrolidone, and the like. Reactive diluent systemsare well known to those skilled in the art of radiation curing and theselection of an appropriate diluent system in any given instance issufficiently encompassed by such knowledge as to require no furtherdiscussion here.

It has also been found that the inclusion of chain transfer agents inthe energy-curable compositions employed in the practice of thisinvention can beneficially affect ultimate cured film properties.Substantially any of the known chain transfer agents can be so employed.Generally, such compounds, when utilized, will be employed at levels notexceeding about 15 parts by weight, per 100 parts of combined weight ofunsaturated urethane oligomer and reactive diluent, and preferably willbe employed in the range from about 0.1 to about 5 parts by weight.Representative chain transfer agents for addition polymerizationreactions include benzene; toluene; ethylbenzene, isopropylbenzene,t-butylbenzene; cyclohexane; heptane; n-butyl chloride; n-butyl bromide;n-butyl iodide; n-butyl alcohol; n-butyl disulfide; acetone; aceticacid; chloroform; carbon tetrachloride; carbon tetrabromide; butylamine;triethylamine; t-butyl mercaptan; n-butyl mercaptan; tertiary aliphaticamines such as triethanolamine and t-butyl diethanolamine;2-ethylhexane-1,3-dithiol; 1,10-decanedithiol; 1,2-ethanedithiol;1,3-propanedithiol; 1,6-octanedithiol; 1,8-octanedithiol;1,10-octadecanedithiol; m-benzenedithiol; bis-(2-mercaptoethyl) sulfide;p-xylylenedithiol; pentaerythritol tetra-7-mercaptoheptanoate;mercaptoacetic acid triglyceride; pentanethiol; dodecanothiol; glycolmercaptoacetate; ethyl mercaptoacetate; and esters of thioglycolic andmercaptopropionic acids. Preferred chain transfer agents include bothmonothiols and polythiols; the polythiols having a molecular weight inthe range from about 95 to about 20,000 and having the general formula

    R.sup.10 (SH).sub.p,

wherein R¹⁰ is a polyvalent organic moiety and p is at least 2, beingespecially preferred. Particularly preferred polythiols include glyceroltrithioglycolate; pentaerythritol tetrathioglycolate; pentaerythritoltetrakis (β-mercaptopropionate); trimethylolpropane tris(thioglycolate);trimethylolpropane tris(β-mercaptopropionate); ethylene glycolbis(thioglycolate); ethylene glycol bis (β-mercaptopropionate) andpoly(propylene oxide ether) glycol bis(β-mercaptopropionate).

Preferably, the coating compositions of the invention will also containfrom about 0.1 to about 10 parts by weight, per 100 parts combinedweight of unsaturated oligomer and reactive diluent, of acrylic acid.

The invention compositions can also include pigments, fillers, wettingagents, flow control agents, and other additives typically present incoating compositions. In some applications, the inclusion of minoramounts of inert solvents can be advantageous. Such additive materialsare well known to those skilled in the art and do not require furtherelaboration herein. Also well known are the concentrations at which suchadditives are used.

The coating compositions of this invention are prepared by conventionalmethods such as blending. The compositions can be applied to wood,metal, fabric and plastic substrates in an economical and efficientmanner using conventional industrial techniques and provide smooth,uniform films which are rapidly cured to dried films having excellentphysical and chemical properties.

The coating compositions of this invention can be applied and cured byany of the conventional known methods. Application can be by rollcoating, curtain coating, airless spray, dipping or by any otherprocedure. The cure can be effected by exposure to actinic radiation,especially ultraviolet light radiation. The equipment utilized forcuring, as well as the appropriate time for curing, and the conditionsunder which the curing is effected are well known to those skilled inthe art of radiation curing and do not require further elaborationherein. The herein described coating compositions are rapidly cured tohard mar-resistant films under inert atmospheres. The compositionscontaining the photocatalyst systems are especially suited for curing inthe presence of molecular oxygen at a rate at least equivalent to thatobtained under inert atmospheres. Generally, the cured films willprovide a high gloss finish. Low gloss finishes can be obtained byemploying the herein described compositions containing the aromaticacyloin urethan compounds in the process of Hahn U.S. Pat. No.3,918,393, the disclosure of which is incorporated herein by reference.Compositions containing the novel photocatalyst systems of the inventionare especially suited for obtaining low gloss finishes by the recentlydeveloped "Gradient Intensity Cure" method. According to this method, aflatted or low gloss finish is produced by subjecting thephotopolymerizable composition to actinic light in an oxygen-containingatmosphere at a first intensity level and a first exposure time untilthe composition is completely cured except for the surface thereof andsubsequently subjecting such composition having such uncured surface toactinic light at a second intensity level and a second exposure time tocompletely cure said surface, wherein said combination of secondintensity and second exposure time is selected from the group consistingof

(i) said second intensity is substantially equal to said first intensityand said second exposure time is greater than said first exposure time;

(ii) said second intensity is greater than said first intensity, andsaid second exposure time is substantially equal to said first exposuretime;

(iii) said second intensity is greater than said first intensity, andsaid second exposure time is less than said first exposure time; and

(iv) said second intensity is greater than said first intensity, andsaid second exposure time is greater than said first exposure time.

In accordance with the "Gradient Intensity Cure" method, the coating isfirst irradiated by actinic light in an oxygen-containing atmosphere,with air being the preferred atmosphere, at a first intensity levelwhich is sufficient to energize the photoinitiator component of thephotocatalyst system and initiate free radical polymerization of thebulk of the coating. While actinic radiation has an emission spectrawhich is sufficient to energize also the photosensitizer component ofthe photocatalyst system, both the amount of photosensitizer and thefirst intensity level are selected to ensure that the free radicalsproduced from such energizing of the photosensitizer are insufficient tooverride completely oxygen inhibition at the film surface. The surfaceof the coating is thus inhibited at the first intensity level by theoxygen present in the curing atmosphere at least to the extent that thesurface is incompletely polymerized and remains wet to the touch whilethe bulk or underneath portion of the coating is cured to a hardpolymer.

Following the exposure at the first intensity level, the wet film isirradiated by actinic light in an oxygen-containing atmosphere, with airagain being the preferred atmosphere, at a second intensity level whichis not only higher than that initially employed but also is effective toenergize the photosensitizer component of the photocatalyst system. Thissecond intensity level must be sufficiently high to ensure that thegross amount of free radicals resulting from such energization ofphotosensitizer is effective to override oxygen inhibition at the filmsurface and initiate free radical polymerization of and effect completecure of the wet surface layer. Properties such as stain, solvent andabrasion resistance are substantially identical in comparison toformulations cured according to the two-stage air-inert environmentprocess of Hahn U.S.A. Pat. No. 3,918,393, or cured in a single stage atconstant intensity in either an inert atmosphere or an oxygen-containingatmosphere.

The actinic energy which is preferred for curing the herein-describedcoating composition is ultra violet light or irradiation in the near andfar ultraviolet spectrum, i.e., having wavelengths in the range of 200nm (nanometers) to 400 nm. Various sources of such ultraviolet light orradiation are available in the art including, by way of example, mercuryvapor arc lamps, carbon arcs, plasma arc torches, ultraviolet lasers,and pulsed xenon lamps, with medium pressure mercury arc vapor lampsbeing currently preferred.

The invention is illustrated in greater detail by the followingExamples, but these examples are not to be construed as limiting thepresent invention. All parts, percentages and the like are in parts byweight, unless otherwise indicated.

EXAMPLE I

A reaction vessel equipped with temperature sensing means, coolingmeans, reflexing means and agitation means is charged with 21.2 gbenzoin, 122.5 g tetrahydrofuran and 0.02 g dibutyltin dilaurate. Tothis reaction mixture there is added 19.7 g tosyl isocyanate in asequential manner so as to maintain the reaction temperature below 40°C. The reaction is terminated at an --NCO content below 0.1%. Thereaction product is a strong yellow solution at a 50% total solidscontent. The reaction product is recrystallized from tetrahydrofuran andconfirmed as a 2-p-toluene sulfonyl-urethan-1,2-diphenyl ethanone (A¹and A² are phenyl, R⁰ is hydrogen, Q is ##STR9## m is 1, and n is zero).

By elemental analysis:

    ______________________________________                                                  Percent                                                             Element     Theoretical    Found                                              ______________________________________                                        C           64.62          64.40                                              H           4.66           4.88                                               N           3.42           3.41                                               S           7.77           7.80                                               O           19.53          19.51                                                          100.00         100.00                                             ______________________________________                                    

EXAMPLE II

Analogously to Example 1, 0.1 mol benzoin and 0.05 mol toluenediisocyanate give a mixture of ##STR10## (A¹ and A² are phenyl, R⁰ ishydrogen, Q is ##STR11## m is 2 and n is zero).

EXAMPLE III

Following the procedure of Example I, an acyloin urethan compound havingthe formula ##STR12## (A¹ and A² are phenyl, R⁰ is hydrogen, Q is##STR13## m is 2 and n is zero) is prepared by reacting 0.2 mol benzoinand 0.1 mol 4,4'-methylenebis(phenyl isocyanate) in tetrahydrofuran at25% total solids content (TSC).

EXAMPLE IV

Analogously to Example I, 0.2 mol benzoin and 0.1 mol4,4'-methylene-bis-(cyclohexyl isocyanate) in dimethylformamide at 50%TSC are reacted to form an acyloin urethan compound having the structure##STR14## (A¹ and A² are phenyl, R⁰ is hydrogen, Q is ##STR15## m is 2and n is zero).

EXAMPLE V

Following the procedure of Example I, 0.2 mol benzoin and 0.1 toluenediisocyanate are reacted in tetrahydrofuran at 50% TSC to an endpoint of2.70% residual isocyanate content. To the reaction mixture there isadded 0.2 mol 2-hydroxyethyl acrylate and the reaction is terminatedwhen the residual isocyanate content is essentially 0%. The acyloinurethan reaction product comprises a mixture of compounds having theformulas ##STR16##

EXAMPLE VI

Analogously to Example I, 0.1 mol benzoin, 0.1 mol4,4'-methylene-bis-(cyclohexyl isocyanate) and 0.1 mol hydroxypropylacrylate are reacted in 2-ethylhexyl acrylate at 33.3% TSC to give anacyloin urethan reaction product having the formula ##STR17##

EXAMPLE VII

Following the procedure of Example I, 0.1 mol4,4'-methylene-bis-(cyclohexyl isocyanate) and 0.1 mol hydroxypropylmethacrylate at 100 TSC are contacted to form an isocyanate-functionaladduct. To the reaction mixture containing such isocyanate-functionalunsaturated adduct there is added 0.1 mol benzoin. The resultingreaction product is an acyloin urethane compound having the formula##STR18##

EXAMPLE VIII

Following the procedure of Example I, 0.1 mol isophorone diisocyanateand 0.2 mol benzoin are reacted in tetrahydrofuran at 25% TSC to affordthe acyloin urethan compound having the structure ##STR19## (A¹ and A²are phenyl, R⁰ is hydrogen, ##STR20## m is 2 and n is zero).

EXAMPLE IX

Analogously to Example I, 21.2 parts by weight benzoin and 26.7 parts byweight hexamethylene diisocyanate biuret (NCO:OH::1:1) are reacted intetrahydrofuran to give an acyloin urethan compound having the structure##STR21## (A¹ and A² are phenyl, R⁰ is hydrogen, ##STR22## m is 3 and iszero).

EXAMPLE X

Following the procedure of Example I, 0.1 mol furoin and 0.1 molp-toluenesulfonyl isocyanate (tosyl isocyanate) are reacted intetrahydrofuran at 33.3% TSC to give the acyloin urethan compound havingthe formula ##STR23## (A¹ and A² are furyl, R⁰ is hydrogen, Q is##STR24## m is 1 and n is zero).

EXAMPLE XI

Photosensitizer photoinitiator blends are prepared according to theinventive concept as follows:

Following the procedure of Example I, one mol4,4'-methylene-bis-(cyclohexyl isocyanate) and 2 mols benzoin arereacted in situ in benzophenone at 25% TSC to give a mixture of theacyloin urethan photoinitiator having the formula ##STR25## (A¹ and A²are phenyl, R⁰ is hydrogen, Q is ##STR26## m is 2 and n is zero) inbenzophenone photosensitizer.

Alternatively, and preferably, photoinitiator-photosensitizer blends areprepared by mechanically blending, as by ball mixing, the acyloinurethan compound photoinitiator prepared by the procedure of Example IVwith benzophenone photosensitizer in ratios, by weight, ofphotoinitiator:photosensitizer of 1:1, 1:2, 1:3, and 1:4.

EXAMPLE XII

An unsaturated oligomer syrup is prepared by reacting 1 mol of polyesterpolyol (1,3-butylene glycol/glycerine/adipic acid/isophthalic acidcondensation product) having a hydroxyl functionality of 2.3 and 3.5mols 4,4'-methylene-bis (cyclohexyl isocyanate) in 2-ethylhexyl acrylatediluent. The resulting isocyanate-functional oligomer is fully cappedwith 2-hydroxyethyl acrylate to afford a syrup of addition-polymerizableunsaturated oligomer in 2-ethylhexyl acrylate reactive monomer diluentat 70 percent resin solids. The unsaturated oligomer has a molecularweight ca. 1,300 and approximately 1.8 units of vinyl unsaturation per1000 units of molecular weight. The syrup is identified hereinafter asSyrup A.

In an analogous manner, 1 mol of poly(tetramethylene oxide) diol isreacted with an excess of 4,4'-methylene-bis(cyclohexyl isocyanate) in2-ethylhexyl acrylate to form an isocyanate-functional prepolymer in2-ethylhexyl acrylate. The prepolymer is reacted with 2-hydroxyethylacrylate in the presence of 2-ethylhexyl acrylate reactive monomerdiluent to form a syrup of acrylated urethane oligomer having at leasttwo terminal ethylenically unsaturated groups and substantially no freeisocyanate functions, in 2-ethylhexyl acrylate reactive monomer diluentat 70 percent resin solids. The syrup is identified hereinafter as SyrupB.

EXAMPLE XIII

A series of coating compositions are prepared as follows

    ______________________________________                                        Composition   A      V      C    D    E    F                                  ______________________________________                                        Syrup B       100    100    100  100  100  100                                2-Ethylhexyl acrylate                                                                        15     15     15   15   15   15                                Benzophenone  3      3      0    3    3    3                                  Benzoin isobutyl ether                                                                      1      --     --   --   --   --                                 Acyloin urethan                                                               Example IV    --     1      1    --   --   --                                 Example V     --     --     --   1    --   --                                 Example I     --     --     --   --   2    --                                 Example X     --     --     --   --   --   3                                  ______________________________________                                         The compositions are coated onto aluminum substrates to provide a wet film     thickness of 1.5 mil. The thus-coated substrates are cured by exposure to     ultraviolet light at an intensity of 80 W/cm at a line speed of 15.2     meters/minute. All compositions cure in a nitrogen environment to a     mar-resistant surface in one pass. In an oxygen environment the     state-of-the-art control composition A (U.S. Pat. No. 4,017,652) and     invention compositions B and E cure to a mar-resistant surface in one     pass. Invention composition D requires two passes for cure in an oxygen     environment, while invention composition C, which contains no     photosensitizer requires three passes for cure in an oxygen environment,     and invention composition F requires five passes for cure in an oxygen     environment. Film properties of all compositions, whether cured in     nitrogen or oxygen, are substantially identical. With the exception of     composition F, all cured films are clear, that is, not discolored.

EXAMPLE XIV

Using Syrup A of Example XII, photocurable compositions are preparedfrom the following ingredients:

    ______________________________________                                        Composition            A        B                                             ______________________________________                                        Syrup A                79.5     79.5                                          2-Ethylhexyl acrylate  7.9      7.9                                           Silica                 8.2      8.2                                           Acrylic acid           1.6      1.6                                           ν-Methacryloxypropyltrimethoxy silane                                                             0.6      0.6                                           Benzophenone           1.0      1.0                                           Benzoin isobutyl ether 1.5      --                                            Acyloin urethane of Example I                                                                        --       1.5                                           ______________________________________                                    

The resulting coating compositions are applied by direct roll coater tovinyl sheet goods. The coatings are cured by exposure to ultravioletirradiation as follows:

(1) in air using a source consisting of 2 medium pressure mercury vaporlamps at a first intensity of 40 watts/cm at a transport speed of 10meters/minute and then to a second and higher intensity of 80 watts/cmprovided by a power source consisting of 3 medium pressure mercury vaporlamps at a transport speed of 10 meters/minute;

(2) in nitrogen using a power source consisting of 2 medium pressuremercury vapor lamps at an intensity of 40 watts/cm at a transport speedof 10 meters/minute;

(3) same as (2), except that the power source consists of 3 mediumpressure mercury vapor lamps at an intensity of 80 watts/cm;

(4) in air using a power source consisting of 2 medium pressure mercuryvapor lamps at an intensity of 40 watts/cm at a transport speed of 10meters/minute; and then in nitrogen using the same power source at thesame transport speed; and

(5) same as (4), except that the power source consists of 3 mediumpressure mercury vapor lamps at an intensity of 80 watts/cm.

The physical properties of the coatings are substantially equivalent.The coatings cured by methods 1 (gradient intensity method of glosscontrol) and 4 (dual cure method of gloss control, Hahn U.S. Pat. No.3,918,393) have a reduced gloss of 45-50 as measured by the 60° glossmeter, while the coatings cured by methods 2, 3, and 5 have a high glossfinish.

The 60 gloss meter test is a standard test for gloss wherein light isreflected off the coating at a 60° angle and the percent reflectance ismeasured. The test procedure is fully detailed in ASTM standardD-523-67.

EXAMPLE XV

Photosensitizer/photoinitiator blends prepared according to the in situcook method and mechanical blending methods of Example XI at a ratio of1 part by weight photoinitiator to 3 parts by weight photosensitizer areemployed to prepare the following coating compositions:

    ______________________________________                                        Composition             A      B      C                                       ______________________________________                                        Syrup B                 100    100    100                                     2-Ethylhexyl acrylate   7      7      7                                       Pentaerythritol triacrylate                                                                           5      5      5                                       Benzophenone            3      1.5    1.5                                     Benzoin isobutyl ether  1      --     --                                      In situ cook blend of benzophenone                                                                    --     2      --                                      photosensitizer and benzoin/4,4'-methylene-                                   bis-(cyclohexyl isocyanate) acyloin urethan                                   photoinitiator                                                                Mechanical blend of benzophenone photo-                                                               --     --     2                                       sensitizer and benzoin/4,4'-methylene-bis-                                    (cyclohexyl isocyanate) acyloin urethan                                       photoinitiator                                                                ______________________________________                                    

The compositions are coated onto aluminum substrates to provide a wetfilm thickness of 1.5 mil. The coated substrates are cured by exposurein an oxygen atmosphere to ultraviolet light at an intensity of 80watts/cm at a line speed of 15.2 meter/minute, employing 3 mediumpressure mercury vapor lamps as a power source. The compositions cure tocolorless mar-resistant surfaces in one pass.

EXAMPLE XVI

The acyloin urethane compositions of Examples 1 to 10 are employed toprepare coating compositions as follows:

    __________________________________________________________________________    Composition   1  2  3  4  5  6  7  8  9  10 11                                __________________________________________________________________________    Syrup B       100                                                                              100                                                                              100                                                                              100                                                                              100                                                                              100                                                                              100                                                                              100                                                                              100                                                                              100                                                                              100                               2-Ethylhexyl acrylate                                                                       7  7  7  7  7  7  7  7  7  7  7                                 Pentaerythritol triacrylate                                                                 5  5  5  5  5  5  5  5  5  5  5                                 Benzoin isobutyl ether                                                                      1.5                                                                              -- -- -- -- -- -- -- -- -- --                                Acyloin urethane                                                              Example I     -- 1.5                                                                              -- -- -- -- -- -- -- -- --                                Example II    -- -- 1.5                                                                              -- -- -- -- -- -- -- --                                Example III   -- -- -- 1.5                                                                              -- -- -- -- -- -- --                                Example IV    -- -- -- -- 1.5                                                                              -- -- -- -- -- --                                Example V     -- -- -- -- -- 1.5                                                                              -- -- -- -- --                                Example VI    -- -- -- -- -- -- 1.5                                                                              -- -- -- --                                Example VII   -- -- -- -- -- -- -- 1.5                                                                              -- -- --                                Example VIII  -- -- -- -- -- -- -- -- 1.5                                                                              -- --                                Example IX    -- -- -- -- -- -- -- -- -- 1.5                                                                              --                                Example X     -- -- -- -- -- -- -- -- -- -- 1.5                               __________________________________________________________________________

The compositions are coated onto aluminum substrates to provide a wetfilm thickness of 1.5 mil. The coated substrates are cured by exposureunder a nitrogen atmosphere to ultraviolet irradiation, employing as apower source 3 medium pressure mercury vapor lamps, at an intensity of80 watts/cm at a line speed of 15.2 meters/minute. All compositions cureto mar-resistant surfaces in one pass.

EXAMPLE XVII

Analogously to Example XVI, coating compositions which are identical tothose of Example XVI except that each contains 3 parts by weightbenzophenone. The compositions are coated onto aluminum panels andsubjected to ultraviolet irradiation in an oxygen-containing atmosphere,employing the same cure conditions as in Example XVI. All compositionscure to a mar-resistant finish having a high gloss in one pass.

EXAMPLE XVIII

Photocurable compositions are prepared from the following ingredients:

    ______________________________________                                        Composition            A        B                                             ______________________________________                                        Syrup B                100      100                                           2-Ethylhexyl acrylate   17       17                                           Pentaerythritol triacrylate                                                                          5        5                                             Benzophenone           3        1.5                                           Benzoin isobutyl ether 1        --                                            In situ cook blend of benzophenone                                                                   --       2.0                                           photosensitizer and benzoin/4,4'-                                             methylene-bis-(cyclohexyl isocyanate)                                         photoinitiator at a 2:1 ratio                                                 ______________________________________                                    

The compositions are stored at 60° C. Gelation of composition A occursafter 12 days; whereas gelation of composition B requires more than 25days.

The data demonstrate the ability of the novel photoinitiators to effectcuring upon exposure to ultraviolet irradiation in both inert andoxygen-containing environments to afford products possessing propertiesat least equivalent to those obtained by the prior art.

We claim:
 1. A photocatalyst system consisting essentially of(a) atleast one photosensitizer which is effective to promote free radicalpolymerization through bimolecular photochemical reactions of the energydonor type, the hydrogen abstraction type, or by formation of adonor-acceptor complex with monomers or additives leading to ionic orradical species; and (b) at least one photoinitiator which promotes freeradical polymerization by generating reactive specie by way ofunimolecular homolysis resulting from photoexcitation, saidphotoinitiator having the formula ##STR27## wherein A¹ and A² are thesame or different and each is a benzenoid or nonbenzenoid aromaticradical; R⁰ is hydrogen, or an aliphatic, cycloaliphatic or aromaticradical containing from 1 to 9 carbon atoms, or--OR², or ##STR28## R¹ ishydrogen or an aliphatic, cycloaliphatic or aromatic radical containingfrom 1 to 9 carbon atoms, or --R³ --O--R⁴, or ##STR29## R² is hydrogen,or an aliphatic, cycloaliphatic or aromatic radical having from 1 to 9carbon atoms, or --R³ --O--R⁴, or ##STR30## R³ is a divalent aliphatic,cycloaliphatic or aromatic radical containing from 1 to 9 carbon atoms;R⁴ is a hydrogen, or an aliphatic, cycloaliphatic or aromatic radicalhaving from 1 to 9 carbon atoms, or ##STR31## Q is the monovalent orpolyvalent residue of a monomeric aliphatic, cycloaliphatic or aromaticisocyanate compound having at least one isocyanate group, which residueremains after reaction of all isocyanate groups of such monomericcompound with at least one saturated or unsaturated compound containingat least one isocyanate-reactive active hydrogen atoms; X is selectedfrom the group consisting of --O--, --S--, and >N--R⁵, wherein R⁵ ishydrogen or an alkyl group having from 1 to 9 carbon atoms; Y ishydrogen or the organic residue of a saturated or unsaturated organiccompound containing at least one isocyanate-reactive active hydrogenatom, which residue remains after reaction of at least one such activehydrogen atom with an isocyanate moiety; m is a number in the range from1 to f, wherein f is the total number of isocyanate groups per moleculeof isocyanate compound; and n is equal to f-m.
 2. The photocatalystsystem of claim 1 wherein said photosensitizer is present in an amountin the range from 15 to 90 percent by weight of said photocatalystsystem and said photoinitiator is present in an amount in the range from85 to 10 percent by weight of said photocatalyst system.
 3. Thephotocatalyst system of claim 2 containing from 0.01 to 30 parts byweight, per hundred parts by combined weight of said photosensitizer andsaid photoinitiator, of at least one thiol selected from the groupconsisting of monothiols and polythiols, said polythiols having amolecular weight in the range from 95 to 20,000 and having the formula##STR32## wherein R¹⁰ is a polyvalent organic moiety and p is at least2.
 4. The photocatalyst system of claim 2 wherein said photosensitizercomprises benzophenone.
 5. The photocatalyst system of claim 1 whereinsaid photosensitizer comprises benzophenone, said photoinitiatorcomprises a compound having the formula ##STR33## and wherein the amountof said photosensitizer is in the range from 15 to 90 percent by weight,based on total weight of said photocatalyst system, and the amount ofsaid photoinitiator is in the range from 85 to 10 percent by weight,based on total weight of said photocatalyst system.
 6. The photocatalystsystem according to claim 1 wherein said photosensitizer comprisesbenzophenone, said photoinitiator comprises a compound having theformula ##STR34## a mixture thereof; and wherein the amount of saidphotosensitizer is in the range from 15 to 90 percent by weight, basedon total weight of said photocatalyst system, and the amount of saidphotoinitiator is in the range from 85 to 10 percent by weight, based ontotal weight of said photocatalyst system.
 7. The photocatalyst systemaccording to claim 4 wherein each of A¹ and A² is phenyl, R⁰ ishydrogen, Q is ##STR35## m is 2 and n is
 0. 8. The photocatalyst systemaccording to claim 1 wherein said photosensitizer comprisesbenzophenone, said photoinitiator comprises a compound having theformula ##STR36## and wherein the amount of said photosensitizer is inthe range from 15 to 90 percent by weight, based on total weight of saidphotocatalyst system, and the amount of said photoinitiator is in therange from 85 to 10 percent by weight, based on total weight of saidphotocatalyst system.
 9. The photocatalyst system according to claim 4wherein each of A¹ and A² is phenyl, R⁰ is hydrogen, Q is ##STR37## m is2 and n is
 0. 10. The photocatalyst system according to claim 4 whereineach of A¹ and A² is phenyl, R⁰ is hydrogen, Q is ##STR38## m is 3 and nis
 0. 11. The photocatalyst system according to claim 4 wherein each ofA¹ and A² is phenyl, R⁰ is hydrogen, Q is ##STR39## or a mixturethereof, m is 2 and n is 0.